Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
In one prior art approach, a high current, broad ion beam implanter employs a high current density ion source, an analyzing magnet to direct a desired species through a resolving slit and an angle corrector magnet to deflect the resulting beam, while rendering the beam parallel and uniform along its width dimension. A ribbon-shaped ion beam is delivered to a target, and the target is moved perpendicular to the long dimension of the ribbon beam to distribute the ion beam over the target.
Uniform implantation of ions over the surface of the semiconductor wafer is an important requirement in most applications. As semiconductor device geometries decrease in size and wafer diameters increase, device manufacturers demand minimal dose variation over large surface areas. Uniformity is determined, in part, by the profile of the ion beam used for ion implantation. The beam profile is a map of ion beam intensity in a plane orthogonal to the direction of beam transport. The beam current may vary over the cross-sectional area of the ion beam, particularly in the case of large area beams such as ribbon ion beams. Furthermore, the beam profile may vary with implant conditions, such as dopant species, energy and current, and with time. Accordingly, it is desirable to measure and, if necessary, adjust the beam profile in order to enhance ion implanter performance.
A dose measurement and uniformity monitoring system for ion implantation, including a mask plate with sensing apertures and an annular Faraday cup aligned with the apertures, is disclosed in U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr. et al. A beam scanning control device for ion implantation, including a plurality of fixed ion beam detectors, is disclosed in U.S. Pat. No. 4,494,005 issued Jan. 15, 1985 to Shibata et al. An ion beam profile monitor, including a two-dimensional array of sample points placed in the beam, is disclosed by E. P. EerNisse et al. in Rev. Sci. Instrum., Vol. 46, No. 3, (March 1975), pp. 266–268. A method and apparatus for high efficiency scanning in an ion implanter, including a single, slowly-translating Faraday detector, is disclosed in U.S. Pat. No. 4,980,562 issued Dec. 25, 1999 to Berrian et al. All of the prior art beam measuring techniques have had one or more drawbacks, including, but not limited to, low resolution, inaccuracy and slow operation.
Accordingly, there is a need for improved methods and apparatus for ion beam profiling.